For further information, please refer to the US Census Bureau's Annual Services release, online at http://www.census.gov/services/.
Source: US. Bureau of the Census
Release: Annual Services Survey
US. Bureau of the Census, Sources of Revenue: Transportation Infrastructure Engineering Projects for Engineering Services, All Establishments, Employer Firms [REVTIEEF54133ALLEST], retrieved from FRED, Federal Reserve Bank of St. Louis https://research.stlouisfed.org/fred2/series/REVTIEEF54133ALLEST/, August 30, 2015.